The present application is based on Japanese Patent Application No. 2000-388627, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical film which changes the optical path of light incident on a side surface of a liquid-crystal panel into a viewing direction efficiently so that the optical film can be used to form an external light-illumination double mode reflective or transmissible liquid-crystal display device which is thin in thickness and light in weight, and display of which is bright, excellent in uniformity of brightness and easy to view.
2. Description of the Related Art
A liquid-crystal display device smaller in thickness and weight is demanded for the purposes of: suppression of increase in weight of a television set or a personal computer screen because of increase in size thereof; reduction in size and weight of a portable personal computer, a cellular phone, or the like; and so on. For a background-art liquid-crystal display device in which a backlight or a front light using a side light pipe is provided, however, it has been already difficult to reduce the thickness and weight of the liquid-crystal display device any more. It is because a plate thickness is required to be not smaller than 1 mm for light transmission, a light-diffusing plate, a prism sheet, a half mirror, or the like, is required to be disposed to control the optical path and quantity of light, so that a thickness of not smaller than 3 mm is generally required for the background-art liquid-crystal display device. A light pipe including stripe-like light exit means has been proposed. In such a light pipe, however, moire was apt to occur because of interference of the stripe-like light exit means with pixels of a liquid-crystal display panel due to stripe-like light emission. There was therefore a problem that a light-diffusing plate, a prism sheet, or the like, was required for preventing moire. Moreover, such a light pipe needs a linear light source, which is unfavorable for reduction in thickness and weight, so that it was particularly difficult to reduce in thickness and weight of a portable liquid-crystal display device. There was a further disadvantage that a high-grade technique was required for positioning the linear light source relative to the light pipe.
On the other hand, there has been proposed another light pipe in which light exit means constituted by a structure of spherical dots or cylindrical pits in section are provided and light is emitted from a point light source. Because light was reflected by the spherical dots or the cylindrical pits, the reflected light was diffused largely so that the exit light was inclined largely to the normal line (frontal) direction of the light pipe. There was a disadvantage that display was dimmed because the percentage of light entering the liquid-crystal display panel as illumination light was low. There was therefore a problem that a light-diffusing plate, a prism sheet, or the like, was required for eliminating the disadvantage. Accordingly, when an illumination system using a side light pipe is applied to a liquid-crystal display device, thickness and weight of the liquid-crystal display device can be hardly reduced any more than the thickness and weight of an existing liquid-crystal display device because retention members such as a chassis or the like are further required as other constituent members.
It is an object of the present invention to develop an optical film by which the optical path of light, from the a point light source, incident on a side surface of a liquid-crystal display panel is changed into a viewing direction efficiently so that the optical film can be used to form an external light-illumination double mode reflective or transmissible liquid-crystal display device which is thin in thickness and light in weight, and display of which is bright, excellent in uniformity of brightness and easy to view.
According to the present invention, there is provided an optical film constituted by: a transparent film with a refractive index of not lower than 1.49, the transparent film including light exit means which are constituted by concave or convex portions and which are arranged partially like pits in one of opposite surfaces of the transparent film so as to face at least one virtual center formed in one place located on or outside a layer thickness surface at an end of the transparent film, each concave or convex portion having a length of not larger than 200 xcexcm and a depth or height of not larger than 50 xcexcm, each of the light exit means having an optical path changing slope inclined to the film surface of the transparent film at an inclination angle in a range of from 35 to 48 degrees, and an elevational face inclined to the film surface at an inclination angle of not lower than 50 degrees. There is further provided a liquid-crystal display device comprising: a liquid-crystal display panel; and the optical film, the optical film being disposed on one of opposite surfaces of the liquid-crystal display panel so that a surface of the optical film in which the light exit means are provided is located outward.
According to the present invention, a point light source is disposed on a side of the liquid-crystal display panel and on a line containing a virtual center of the light exit means in the optical film to thereby make radiant light enter the panel. Hence, the optical path of the light can be changed, through the optical path changing slopes of the light exit means, into a direction of a surface of the film in which the light exit means are not provided, and with good directivity with respect to the normal line direction of the panel. The optical film can be used in a backlight or a front light to thereby form a transmissible or reflective liquid-crystal display device in which the whole area of the panel display surface can be illuminated uniformly and which is excellent in reduction in thickness and weight and excellent in display quality.
Also in the case where the optical film is disposed on the visual side of the panel to form a front light, external light can be made to enter the optical film efficiently through flat face portions formed between the light exit means partially arranged in the optical film. Hence, liquid-crystal display in an external light mode can be achieved by use of a system in which incident light on a surface of the panel is inverted by a reflection layer disposed in a predetermined position. Hence, an external light-illumination double mode transmissible or reflective liquid-crystal display device as described above can be formed. Moreover, when the arrangement of the light exit means is controlled, the production of moire caused by interference of the light exit means with pixels can be suppressed efficiently. Moreover, the density of arrangement of the optical path changing slopes, or the like, can be adjusted easily, so that the quantity of light transmitted through the optical film in the direction of the thickness of the optical film can be controlled by the area occupied by the light exit means. In addition, the light source can be positioned easily relative to the optical film.